1. Field of the Invention
The present invention relates generally to the field of medical diagnostics, and particularly to the field of determining physiologic characteristics of body lumens. In one particular aspect, the invention provides methods and apparatus for measuring the cross-section of a diseased vessel segment.
To properly treat many bodily diseases or abnormalities, certain physiologic characteristics, such as the size of a particular body member, often need to be determined. One example is in the treatment of vascular lesions, stenosed regions, and particularly vascular aneurysms, which often requires the endoluminal placement of tubular prostheses, such as grafts, stents, and other structures. Before the prothesis is placed in the vascular anatomy, the size of the lesion is measured so that a properly sized prosthesis can be selected.
Vascular aneurysms are defined as the abnormal dilation of a blood vessel, usually resulting from disease and/or genetic predisposition which can weaken the arterial wall and allow it to expand. While aneurysms can occur in any blood vessel, most occur in the aorta and peripheral arteries, with the majority of aortic aneurysms occurring in the abdominal aorta, usually beginning below the renal arteries and often extending distally into one or both of the iliac arteries.
Aortic aneurysms are most commonly treated in open surgical procedures where the diseased vessel segment is by-passed and repaired with an artificial vascular graft. Recently, methods for endovascular graft placement for the treatment of aneurysms have been proposed. One such method and apparatus for endovascular placement of intraluminal prostheses, including both grafts and stents, is described in co-pending U.S. patent application Ser. No. 08/290,021, filed Aug. 12, 1994, the disclosure of which is herein incorporated by reference. A suitable intraluminal prosthesis for such a method includes a radially compressible, tubular frame having a proximal end, a distal end, and an axial lumen therebetween. The prosthesis is delivered to the area of interest via a delivery catheter. The prosthesis is then partially released from the catheter into a blood vessel or other body lumen to allow the prosthesis to radially expand and conform to the interior surface of the lumen being treated. The prosthesis can then be repositioned by the catheter until it is properly placed within the vessel. Optionally, the prosthesis may be implanted within a vessel by expanding a malleable portion of the prosthesis with a balloon catheter. Exemplary graft prostheses are described in co-pending U.S. patent application Ser. No. 08/255,681, the disclosure of which is herein incorporated by reference.
As previously described, before the endoluminal placement of an intraluminal prosthesis, it is desirable to first determine the appropriate size for the expanded prosthesis so that the prosthesis will properly fit within the body lumen. For instance, in the case of vascular aneurysms, it is often desirable to determine the diameter of the vessel adjacent to the aneurysm so that the prosthesis will match the size of the vessel on either side of the diseased area. In other circumstances, the cross-sectional area or the circumference of a lumen would be helpful. For example, where a prosthesis will conform to a vessel which has an irregular cross-section, it is desirable that the periphery of the implanted prosthesis match the lumen circumference to seal along the periphery. Alternatively, the open cross-section area would be helpful in determining whether placement of a prosthesis is an appropriate therapy for a stenosed lumen. As a final example, it is desirable to select a properly sized balloon catheter to firmly implant the prosthesis within the vessel, but which will not over-expand the prosthesis and damage the healthy vessel walls.
Current methods for measuring the open cross section near an effected body lumen employ fluoroscopy. To determine the diameter of a vessel using fluoroscopy, a catheter is inserted into the vessel and a contrast agent is injected into the vessel through the catheter. The blood flow carries the contrast agent along the vessel so that the vessel can be radiographically imaged with a fluoroscope. The fluoroscope produces a planar (or two dimensional) image of the vessel which can be evaluated to determine the existence of a diseased or abnormal area within the vessel. The nominal diameter of the diseased or abnormal area is then estimated by measuring the diameter of the vessel in the area adjacent to the diseased area on the radiographic image. However, such a measurement is typically not particularly accurate since it relies on discerning an ill-defined boundary in a single plane. Such a measurement does not take into account that the vessel is usually not in the same plane as the resulting fluoroscopic image. Another drawback to such procedures in determining the diameter of a vessel is that the vessel is often irregular in cross section, i.e., is not circular. Hence, even if the vessel were in the same plane as the resulting fluoroscopic image of the vessel, it would still be difficult to measure the open diameter of an irregular vessel.
Alternative prior art methods for measuring physiological characteristics of lumens have stressed the diseased lumen being measured. To determine lumen wall compliance and internal diameter, it has been proposed that a balloon be inflated with relatively low pressure fluid within a lumen. By monitoring inflation fluid volume and pressure, wall compliance of an expanding lumen can be determined. By inflating the balloon with sufficient internal pressure to expand the balloon so that it is restrained by the lumen wall, lumen cross-sectional area or diameter can also be measured. However, the balloon must be inflated with sufficient pressure to ensure that it has contacted the lumen wall all along the periphery to obtain an accurate measurment. Additionally, the measurement balloon systems of the prior art have utilized generally cylindrical balloons of non-compliant materials. Hence, the prior art methods have stressed the target region of the diseased lumen by forcing irregular lumens towards a cylindrical shape and by distending the diseased lumen.
Improper determination of the vessel size can result in the selection of a prosthesis that is too small and hence cannot be properly grafted. The endoluminal placement of an improperly sized prosthesis can present a number of serious problems. One problem is that the prosthesis must be removed from the body lumen and replaced with another that is properly sized. This can often be difficult if the prosthesis has been radially expanded while in the body lumen. To remove the expanded prosthesis, the prosthesis must be radially compressed and then withdrawn from the body lumen. Such a procedure increases the risk of injury to the patient as well as unduly increasing operating time and expense.
Methods and apparatus are therefore needed for accurately measuring the cross-section of a body lumen, and in particular the diameter, circumference, and cross-sectional area of a vascular lesion. In one particular aspect, it would be desirable to provide improved methods and apparatus for the measurement of blood vessels in the region adjacent aneurysms so that the proper size of intraluminal prostheses, such as grafts and stents, can be accurately determined. It would be further desirable if such methods and apparatus were simple to use and could be used with existing fluoroscopy technology. Finally, it would be particularly desirable if such measurements could be taken without causing unnecessary stress to the diseased vessel.
2. Description of the Background Art
As previously described, methods and apparatus for placement and repositioning of intraluminal prostheses are described in U.S. patent application Ser. No. 08/290,021, the disclosure of which has previously been incorporated by reference. Suitable graft structures for placement in body lumens are described in U.S. patent application Ser. No. 08/255,681, the disclosure of which has previously been incorporated herein by reference.
U.S. Pat. No. 5,275,169 describes methods and apparatus for determining the internal cross-sectional area and compliance of a vessel by measuring the volume and pressure of an incompressible fluid within an inflated balloon catheter. U.S. Pat. No. 4,651,738 describes a system for monitoring the pressure-volume relationship during conventional angioplasty procedures. U.S. Pat. No. 5,171,299 describes a similar apparatus which displays balloon pressure and diameter based on internal balloon pressure during angioplasty. U.S. Pat. No. 5,135,488 describes an angioplasty system having a microprocessor for controlling, monitoring, displaying, and recording balloon inflation data. The medical literature also describes such measurements. See, for example, Abele (1980) AJR 135:901-906; Dembe et al. (1991) J. Am. Coll. Cardiol. 18:1259-1262. The use of computer enhanced radiographic imaging techniques for determining vascular lumen diameter is described in Serruys et al. (1984) Am. J. Cardiol. 54:482-488; and Nicols et al. (1984) Circulation 69:512-522.